The invention relates to a device for the variable adjustment of control timing of gas exchange valves of an internal combustion engine with a drive element, an output element, at least one pressure chamber, and a volume accumulator, with the output element being arranged rotatably in reference to the drive element, and the pressure chamber being partially limited by said parts, with a phase position between the drive element and the output element being adjustable in a variable fashion by supplying and/or removing pressure medium from the pressure chamber, with pressure medium lines being provided via which pressure medium can be supplied to the pressure chamber or from which it can be removed, with the volume accumulator comprising at least one housing and a separating element that can be displaced therein, which separates a supply chamber communicating with the pressure medium lines from a ventilated complementary chamber, with by the impingement of pressure medium from the supply chamber, the separating element is displaced in the housing such that the volume of the supply chamber increases at the expense of the complementary chamber.
In modern internal combustion engines devices are used for variably adjusting the control timing of gas exchange valves, in order to allow variable designing of the phase position of a camshaft in reference to a crankshaft within a defined angular range between a maximum early and a maximum late position. For this purpose, the device is integrated in a drive train by which torque is transferred from the crankshaft to the camshaft. This drive train can be realized as a belt, chain, or gear drive, for example.
Such a device is known for example from DE 10 2007 041 552 A1. The device comprises a phase position device with a drive element which is in a drive connection with the crankshaft, and which is connected to an output element connected to the camshaft in a torque-proof fashion. Furthermore, the phase position device comprises several pressure chambers with each of the pressure chambers being divided by an impeller into two mutually influencing pressure chambers. By the supply of pressure medium to and/or draining of pressure medium from the pressure chambers, the blades inside the pressure chambers are displaced, leading to a change of the phase position between the output element and the drive element.
The supply of pressure medium to and/or the pressure medium drained from the pressure chambers is controlled via a hydraulic circuit comprising a pressure medium pump, a pressure medium reservoir, a control valve, and several pressure medium lines. The control valve comprises several pressure medium connectors, with one pressure medium line connecting the pressure medium pump to the control valve. One additional pressure medium line each connects one of the operating connections of the control valve to one group each of the oppositely operating pressure chambers.
During operation of the internal combustion engine alternating torques impinge the camshaft, each of which is caused by the cams rolling over gas exchange valves pre-stressed by a valve spring. These torques are transferred to the phase adjustment device and act by braking and/or accelerating the adjustment speed of the phase position. Here, when opening the gas exchange valve a phase adjustment occurs in the direction of a later control timing and when closing the gas exchange valve a phase adjustment is supported in the direction of an earlier control timing. During the phases in which the alternating torques support the phase adjustment the need for pressure medium of the phase adjustment device increases rapidly and can exceed the volume flow rate transported by the pressure medium pump. In this case, the phase adjustment speed is not determined by the supporting alternating torque but by the volume flow rate of the pressure medium pump. In this case, the supporting contribution of the alternating torques cannot be fully utilized. The smaller the dimensioning of the pressure medium pump the more distinct is this negative effect.
In each operating phase of the internal combustion engine the volume flow rate required by the phase adjustment device in order to ensure the function of the phase position adjustment the pressure medium pump must supply. For this purpose a volume accumulator is provided in DE 10 2007 041 552 A1, which communicates with the pressure medium line connecting the pressure medium pump with the control valve. In phases in which the pressure medium required by the phase adjustment device is lower than the volume of pressure medium transported by the pressure medium pump, the volume accumulator is filled. Here, a piston inside a housing is displaced against a force accumulator, in the exemplary embodiment shown a pressure spring. In these phases the force accumulator is stressed, i.e. the force accumulator accepts an amount of energy which can be resupplied to the hydraulic system. When the system pressure falls, the volume accumulator empties under the influence of the relaxing force accumulator into the hydraulic circuit and thus supports the phase adjustment of the camshaft in reference to the crankshaft.
In the exemplary embodiment shown it is disadvantageous that due to the stressed pressure spring, the emptying of the volume accumulator begins to occur already at a point of time at which the volume flow rate supplied by the pressure medium pump is sufficient to ensure the adjustment. The volume of pressure medium displaced in this phase from the volume accumulator is no longer available in the phases of insufficient supply of the phase adjustment device.